(c)
We have, $s = t^{3} - 3t^{2}$
On differentiating with respect to $t$, we get
$\frac{ds}{dt} = 3t^{2} - 6t$ …(i)
Again differentiating Eq. (i), we get
$\frac{d^{2}s}{dt^{2}} = 6t - 6 = 0\ \ \Rightarrow \ \ t = 1$
On putting the value of $t = 1$ in Eq. (i), we get
$\frac{ds}{dt} = 3 \times 1 - 6 \times 1 = 3 - 6 = - 3\ m/s$